Photosensitive letterpress printing original plate

ABSTRACT

The present invention provides a photosensitive letterpress printing original plate provided with a heat sensitive mask layer having high light blocking effect and durability yet prepared as a thin film. A photosensitive letterpress printing original plate including at least (A) a supporting member, (B) a photosensitive resin layer, and (C) a heat sensitive mask layer that are successively laminated, wherein the heat sensitive mask layer (C) contains carbon black and, as a dispersion binder therefor, a butyral resin as well as polyamide containing polar group selected from the group consisting of polyamide containing a tertiary amine group, polyamide containing a quaternary ammonium salt group, polyamide containing an ether group and polyamide containing a sulfonic group.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a photosensitive letterpress printing original plate to be used for producing a letterpress printing plate according to a Computer to Plate Technique, and in particular, it concerns a photosensitive letterpress printing original plate provided with a heat sensitive mask layer having high light blocking effect and durability even if prepared as a thin film.

BACKGROUND ART

In recent years, in the field of letterpress printing, a Computer to Plate Technique (CTP Technique) , known as a digital image-forming technique, has been widely used as a general technique. The CTP technique is a method in which information processed on a computer is directly outputted onto a printing plate so that a concave/convex pattern to forma relief is obtained. This technique eliminates the necessity of negative film producing processes, thereby making it possible to reduce costs and time required for forming the negative film.

In the CTP technique, the negative film, conventionally used so as to cover areas that should not be polymerized, is replaced by a mask that is formed and integrated within a printing plate. With respect to the method for obtaining the integration mask, a method has been widely used in which an infra-red ray sensitive layer (a heat sensitive mask layer) that is opaque to chemical rays is formed on a photosensitive resin layer and by evaporating this infra-red ray sensitive layer by using an infrared laser, an image mask is formed (cf. Japanese Patent Application Republication No. 506201/95).

As the heat sensitive mask layer, those layers, made from carbon black serving as an opaque material to radiations and a binder, are generally used. The heat sensitive mask layer is subjected to abrasion by an infrared laser. Therefore, the layer is desirably made thinner from the viewpoint of abrasion efficiency. Here, as the layer is made thinner, influences of the caused wrinkles onto the relief become lesser. However, in order to prevent chemical radiations from being transmitted into a photo-polymerizable layer, the photosensitive mask layer generally needs to have a transmittance optical density (light blocking effect) of 2.0 or more. An ideal heat sensitive mask layer is such a layer as to provide a predetermined transmittance optical density (light blocking effect) or more yet prepared as a thin film.

In general, the optical density (light blocking effect) of the heat sensitive mask layer is defined by the following formula. Optical density=log (100/T)=εcl

Here, T represents a transmittance (%), c represents a density of an infra-red ray absorbing substance (mol·l⁻¹), l represents a thickness (cm), and ε represents a molecular extinction coefficient (l·mol⁻¹·cm⁻¹). As can be understood by the above-mentioned formula, the optical density can be enhanced by increasing the density of the infrared-ray absorbing material (carbon black) ; however, in this case, the film becomes too brittle, which causes innumerable scratches in working process. The scratched portions fail to shield light, thereby causing unnecessary images to be formed on a relief. Moreover, although the optical density can be increased by making the coat film thickness thicker; however, in this case, high energy is required to cause abrasion, and wrinkles tend to be caused on the relief . Wrinkles on the relief become in particular a serious problem in the case of a water developing plate whose photosensitive resin layer is flexible.

Here, it has been known that the light blocking effect is changed depending on the dispersing state of carbon black even when the carbon black density and the layer thickness are the same. In general, the better the dispersing state, the higher the optical density becomes. Here, the dispersing state of carbon black is highly influenced by a binder resin.

Up to now, as the binder resin for the heat sensitive mask layer, a nylon resin (cf. Japanese Patent Application Laid-Open (JP-A) No. 305030/96) , a polyvinyl alcohol resin (cf. Japanese Patent Application Laid-Open (JP-A) No. 171247/97), a heat decomposable polymer (cf. Japanese Patent Application Laid-Open (JP-A) No. 2002-214792) or a plurality of polymers have been used. In the case where the nylon resin is used, although the durability of the heat sensitive mask layer becomes superior, the dispersing state of carbon black becomes undesirable, causing the necessity of increasing the layer thickness so as to achieve a predetermined optical density. On the other hand, in the case where the polyvinyl alcohol resin is used, the resulting defect is that the durability of the film deteriorates. In the case where the heat decomposable polymer is used as well, it is not possible to obtain a satisfactory abrasion efficiency. Therefore, at present, a heat sensitive mask layer having a high optical density (light blocking effect) with superior durability yet prepared as a thin film has not been achieved.

DISCLOSURE OF THE INVENTION Problem that the Invention is to Solve

The present invention has been devised so as to solve the problems with the conventional art, and its objective is to provide a photosensitive letterpress printing original plate having a heat sensitive mask layer that has high light blocking effect and durability yet prepared as a thin film.

Means for Solving the Problem

As a result of intensive studies made to achieve the above-mentioned objective, the inventors consequently have noted that, by using a butyral resin and polyamide containing polar group as a binder for improving the dispersion property for carbon black of a heat sensitive mask layer, desired effects can be obtained, thereby completing the present invention.

That is, the present invention relates to a photosensitive letterpress printing original plate including at least (A) a supporting member, (B) a photosensitive resin layer, and (C) a heat sensitive mask layer that are successively laminated, wherein the heat sensitive mask layer (C) contains carbon black and, as a dispersion binder therefor, a butyral resin as well as polyamide containing polar group selected from the group consisting of polyamide containing a tertiary amine group, polyamide containing a quaternary ammonium salt group, polyamide containing an ether group and polyamide containing a sulfonic group.

In one preferable mode of the photosensitive letterpress printing original plate of the present invention, the butyral resin and the polyamide containing polar group have a weight ratio in a range from 20 to 80:20 to 80, and the carbon black and dispersion binder have a weight ratio in a range from 25 to 50:20 to 75.

Advantages of the Invention

The photosensitive letterpress printing original plate of the present invention makes it possible to improve the dispersing property of carbon black by using a butyral resin as a dispersion binder for carbon black of the heat sensitive mask layer and also to enhance the durability as well as improving the dispersing property due to the polar group by using polyamide containing polar group as a binder for carbon black. Therefore, the present invention makes it possible to achieve a high light blocking effect by using the heat sensitive mask layer with a thin thickness, and consequently to provide a photosensitive letterpress printing original plate that can be subjected to abrasion at low energy, and is less vulnerable to wrinkles on the relief.

BEST MODE FOR CARRYING OUT THE INVENTION

The photosensitive letterpress printing original plate of the present invention will now be illustrated in detail as follows.

The photosensitive letterpress printing original plate of the present invention has a constitution in which at least (A) a supporting member, (B) a photosensitive resin layer, and (C) a heat sensitive mask layer are successively laminated.

The supporting member (A) to be used for the original plate of the present invention is flexible, and preferably made from a material having a superior dimension stability, and examples thereof include: a supporting member made of metal, such as steel, aluminum, copper and nickel, and a supporting member made of a thermoplastic resin, such as a polyethylene terephthalate film, a polyethylene naphthalate film, a polybutylene terephthalate film and a polycarbonate film. Among these, the polyethylene terephthalate film, which has a superior dimension stability and a sufficiently high viscoelasticity, is in particular preferably used. The thickness of the supporting member is set to 50 to 350 μm, preferably, to 100 to 250 μm, from the viewpoints of mechanical properties, shape stability and handling characteristics upon producing a printing plate. Moreover, if necessary, in order to improve an adhesive property between the supporting member and a photosensitive resin layer, an adhesive may be formed between them.

The photosensitive resin layer (B) to be used for the original plate of the present invention is composed of essential components, such as a synthetic polymer compound, a photopolymerizable unsaturated compound and a photopolymerization initiator, and desirable additives, such as a plasticizer, a thermal polymerization inhibitor, a dye, a pigment, an ultraviolet-ray absorbing agent, perfume, and an antioxidant.

As the synthetic polymer compound, conventional soluble synthetic polymer compound may be used, and examples thereof include: polyether amide (cf. Japanese Patent ApplicationLaid-Open (JP-A) No. 79437/80 etc.), polyetherester amide (cf. Japanese Patent Application Laid-Open (JP-A) No. 113537/83 etc.), polyamide containing a tertiary nitrogen atom (cf. Japanese Patent Application Laid-Open (JP-A) No. 76055/75 etc.), polyamide containing a tertiary nitrogen atom of ammonium salt type (cf. Japanese Patent Application Laid-Open (JP-A) No. 36555/78 etc) , addition polymer of amide compound having one or more amide bond and organic diisocyanate compound (cf. Japanese Patent Application.Laid-Open (JP-A) No. 140737/83 etc.), and addition polymer of diamine having no amide bond and organic diisocyanate compound (cf. Japanese Patent Application Laid-Open (JP-A) No. 97154/92 etc.). Among these, the polyamide containing a tertiary nitrogen atom and the polyamide containing a tertiary nitrogen atom of ammonium salt type are preferable.

As the photopolymerizable unsaturated compound, ring-opening addition reaction product of polyglycidyl ether of polyhydric alcohol, methacrylic acid and acrylic acid is exampled. As the polyhydric alcohol, dipentaerythritol, pentaerythritol, trimethylolpropane, glycerin, ethylene glycol, diethylene glycol, triethylene glycol, and ethylene oxide addition product of phthalic acid are exampled. Among these, trimethylolpropane is preferable.

As the photopolymerization initiator, benzophenones, benzoins, acetophenones, benzyls, benzoin alkylethers, benzyl alkylketals, anthraquinones, and thioxanthenes are exampled. Specifically, benzophenone, chlorobenzophenone, benzoin, acetophenone, benzyl, benzoin methylether, benzoin ethylether, benzoin isopropylether, benzoin isobutylether, benzyl dimethylketal, benzyl diethylketal, benzyl diisopropylketal, anthraquinone, 2-ethylanthraquinone, 2-methylanthraquinone, 2-allylanthraquinone, 2-chloroanthraquinone, thioxanthone, and 2-chlorothioxanthone are exampled.

The heat sensitive mask layer (C) to be used for the original plate of the present invention is composed of carbon black that is a material having a function for absorbing infrared laser rays to convert them into heat and a function for blocking ultraviolet rays, and a dispersion binder used for the carbon black. Moreover, desired components except for these, such as a pigment dispersant, a filler, a surfactant or a coating aid, may be contained therein within such a range as not to impair the effects of the present invention.

The present invention is characterized in that a butyral resin and polyamide containing polar group are used in combination as a dispersion binder for carbon black. The butyral resin can improve the dispersing property of the carbon black and contribute to high light blocking effect; however, the use of the butyral resin alone makes the coat film of the heat sensitive mask layer brittle to cause deterioration of durability. Therefore, in order to maintain the durability, the present invention uses the polyamide containing polar group. The polyamide containing polar group is superior in the dispersing property of carbon black because of the effect of the polar group contained therein, and causes no deterioration of the dispersing property even when one portion of the butyral resin is replaced by this. Here, both of the butyral resin and the polyamide containing polar group are dissolved in alcohol and water; therefore, when these are concomitantly used as dispersion binders, it becomes possible to easily produce a heat sensitive mask layer that is superior in handling upon forming a film as well as in developing property by a solvent and water. Since the heat sensitive mask layer, made of the butyral resin and the polyamide containing polar group, is easily dissolved in water, it is in particular preferably used as a heat sensitive mask layer for water developing plates.

The butyral resin to be used for a dispersion binder is also referred to as polyvinyl butyral, and one kind of polyvinyl acetal produced by allowing polyvinyl alcohol and butyl aldehyde to react with each other by using an acid catalyst.

The polyamide containing polar group to be used as the dispersion binder is selected from the group consisting of polyamide containing a tertiary amine group, polyamide containing a quaternary ammonium salt group, polyamide containing an ether group and polyamide containing a sulfonic group.

The weight ratio of the butyral resin and the polyamide containing polar group in the dispersion binder of the heat sensitive mask layer (C) is preferably set in a range from 20 to 80:20 to 80. When the weight ratio of the two components is not within the above-mentioned range, there is a fear that the dispersing property of carbon black and the durability of the coat film of the heat sensitive mask layer might not be achieved in a well-balanced manner.

The weight ratio of the carbon black and the dispersion binder in the heat sensitive mask layer (C) is preferably set in a range from 25 to 50:20 to 75. When the weight ratio of the two components is not within the above-mentioned range, there is a fear that the light blocking effect due to carbon black might not be achieved when the heat sensitive mask layer is applied as a thin film.

The heat sensitive mask layer (C) is preferably designed to have an optical density of 2.0 or more, more preferably, an optical density of 2.0 to 3.0, and most preferably, an optical density of 2.2 to 2.5, relative to chemical rays.

The layer thickness of the heat sensitive mask layer (C) is preferably set to 0.5 to 2.5 μm, more preferably, to 1.0 to 2.0 μm. When the layer thickness is set to the above-mentioned lower limit or more, it is possible to obtain an optical density of not less than a predetermined value, without the necessity of a high coating technique. Moreover, when the layer thickness is set to the above-mentioned upper limit or less, high energy is not required for evaporation of the heat sensitive mask layer, thereby making it possible to provide an advantageous method from the viewpoint of costs.

It is preferable to install a peelable flexible cover film on the heat sensitive mask layer (C) to protect the printing original plate. Examples of the preferable peelable flexible cover film include a polyethylene terephthalate film, a polyethylene naphthalate film and a polybutylene terephthalate film. However, such a protective film is not absolutely necessary.

Although the method for manufacturing a letterpress printing original plate of the present invention is not particularly limited, it is generally manufactured by using the following processes:

First, components of the heat sensitive mask layer, such as a binder, except for carbon black are dissolved in an appropriate solvent, and carbon black is dispersed therein to prepare a dispersion solution. Next, this dispersion solution is applied to a supporting member (for example, a PET film) for a heat sensitive mask layer, and the solvent is evaporated so that one of laminated bodies is formed. In the meantime, in a separated manner from this, a photosensitive resin layer is formed on a supporting member by a coating process so that the other laminated body is prepared. The two laminated bodies, thus obtained, are laminated under pressure and/or a heating process so that the photosensitive resin layer is arranged adjacent to the heat sensitive mask layer. Here, the supporting member for the heat sensitive mask layer functions as a surface protective film after completion of the printing original plate.

When the protective film is present, a method for manufacturing a printing plate from the printing original plate of the present invention includes processes in which, first, the protective film is removed from the photosensitive printing original plate. Thereafter, the heat sensitive mask layer is imagewise-irradiated with an IR laser so that a mask is formed on the photosensitive resin layer. Preferable examples of the IR laser include an ND/YAG laser (1064 nm) and a diode laser (for example, 830 nm) . A laser system desirably used for the Computer to Plate Technique is commercially available, and, for example, a CDI Spark (made by Esko-Graphics Co., Ltd.) may be used. This laser system includes a rotation cylinder drum used for holding a printing original plate, an IR laser irradiating device and a layout computer, and image information is directly transferred from the layout computer to the laser device.

After the image information has been written in the heat sensitive mask layer, active light rays are applied onto the entire surface of the photosensitive printing original plate through the mask. This process may be carried out with the plate attached to the laser cylinder; however, a method is generally used in which, after the plate has been removed from the laser cylinder, the irradiation process is carried out by using a commonly-used irradiation unit having a flat plate shape because this method is more advantageous in that even a plate size out of the standard can be appropriately handled. Examples of the active light rays include: ultraviolet rays having a wavelength in a range from 150 to 500 nm, in particular, from 300 to 400 nm. Examples of its light source include: a low-pressure mercury lamp, a high-pressure mercury lamp, a super-high pressure mercury lamp, a metal halide lamp, a xenon lamp, a zirconium lamp, a carbon arc lamp and an ultraviolet-ray fluorescent lamp. Thereafter, the irradiated plate is developed so that a printing plate is obtained. The developing process can be carried out by using a commonly-used developing unit.

EXAMPLES

The present invention will now be specifically illustrated by way of the following Examples although the present invention is not limited thereto.

Preparation of the Dispersion Binder

As the dispersion binder, a butyral resin, polyamide containing a tertiary amine group, polyamide containing an ether group, polyamide containing a quaternary ammonium salt group, polyamide containing a sulfonic group, polyamide containing no polar group and polyvinyl alcohol were prepared. BM-5 made by Sekisui Chemical Co. , Ltd. was used as the butyral resin. With respect to polyamide containing a tertiary amine group, the polyamide containing an ether group, the polyamide containing a quaternary ammonium salt group and the polyamide containing a sulfonic group, those materials synthesized as described below were used. Macromelt 6900 made by Henkel Japan Ltd. was used as the polyamide containing no polar group. GH23 made by Nippon Synthetic Chemical Industry Co., Ltd was used as polyvinyl alcohol.

Synthesis of the Polyamide Containing a Tertiary Amine Group

Into an autoclave were loaded ε-caprolactam (50 parts by weight), N, N-di (γ-aminopropyl) piperazine adipate (40 parts by weight), 3-bis-aminomethyl cyclohexane adipate (10 parts by weight) and water (100 parts by weight), and after having been substituted with nitrogen, the autoclave was tightly closed and gradually heated. From the time when the inner pressure had reached 10 kg/m³, the water was distilled off until the pressure was no longer maintained, and this was returned to normal pressure in about 2 hours, and then allowed to react with one another at normal pressure for one hour. The maximum polymerization reaction temperature was 255° C. By these processes, polyamide containing a tertiary amine group having a melting point of 137° C. with a specific viscosity of 1.96 was obtained.

Synthesis of the Polyamide Containing an Ether Group

Acrylonitrile was added to each of the two terminals of polyethylene glycol having a number-average molecular weight of 600, and this was subjected to a reduction treatment by hydrogen so that α, ω-diaminopolyoxyethylene was obtained, and an equimolar salt between this and diadipic acid (60 parts by weight), ε-caprolactam (20 parts by weight) and an equimolar salt between hexamethylene diamine and adipic acid (20 parts by weight) were fused and polymerized so that polyamide containing an ether group having a relative viscosity of 2.50 (viscosity obtained by measuring at 25° C. a solution prepared by dissolving 1 g of the polymer in 100 ml of water chloral) was obtained.

Synthesis of the Polyamide Containing a Quaternary Ammonium Salt Group

The polyamide containing a tertiary amine group (100 g), synthesized as described above, was dissolved in 1000 cc of methanol. Successively, adipic acid (4.4 g) was added to this. After having been stirred at 30° C. for 3 hours, methanol was distilled off so that polyamide containing a quaternary ammonium salt group was obtained.

Synthesis of the Polyamide Containing a Sulfonic Group

Into an autoclave were loaded ε-caprolactam (4270 g), hexamethylene di ammonium adipate (2990 g), dimethylisophthalate (1397 g), dimethyl 5-sodium sulfoisophthalate (1397 g), hexamethylene diamine (1253 g) and water (2500 g), and this was subjected to -polycondensation at 130° C. to 270 C for 10 hours so that polyamide containing a sulfonic group was obtained.

Preparation of the Heat Sensitive Mask Layer Coating Solution

In accordance with the composition (weight ratio) described in the heat sensitive mask layer composition of Table 1, a dispersion binder was dissolved in a solvent, and carbon black was dispersed in this solution to prepare a dispersion solution so that a heat sensitive mask layer coating solution was prepared. Here, in Examples 1 to 6 and Comparative Examples 1 to 3, the solvent used here was a mixed solution of methanol and ethanol at a weight ratio of 70:30, in Comparative Example 4, the solvent used here was a mixed solution of n-butanol and toluene at a weight ratio of 80:20, and in Comparative Example 5, the solvent used here was water.

Formation of the Heat Sensitive Mask Layer

The heat sensitive mask layer coating solution was applied to a PET film supporting member (E5000, thickness: 100 μm, made by Toyobo Co. , Ltd.) subjected to a releasing treatment on its two surfaces, by using a bar coater properly selected so as to provide a layer thickness of 1.5 μm, and this was subjected to a drying process at 120° C. for 5 minutes so that a heat sensitive mask layer was formed.

Performance Evaluations

Performances of each of the heat sensitive mask layers thus obtained were evaluated in the following manner.

Light blocking effect: The optical density of the heat sensitive mask layer formed on the PET film supporting member was measured by using a white/black transmission densitometer DM-520 (made by Dainippon Screen MFG Co., Ltd.).

Durability: The heat sensitive mask layer formed on the PET film supporting member was cut into a square piece having a size of 20cm×20cm, and another PET film (E5000, thickness: 100 μm, made by Toyobo Co., Ltd.) was superposed on the layer surface, and while this state is being maintained, these were rubbed against each other once in each of right and left directions without applying any force; then, scratches formed on the surface of the heat sensitive mask layer were inspected by using a 10-fold loupe.

-   ◯: No scratches -   Δ:One to four scratches, each having a length of 50 μm or more, were     observed. -   x: Five or more scratches, each having a length of 50 μm or more,     were observed.

Table 1 shows the results of these performance evaluations.

TABLE 1 The heat sensitive mask layer composition of Examples 1 to 6 and Comparative Examples 1 to 5 and the results of evaluations. Example Comparative Example 1 2 3 4 5 6 1 2 3 4 5 heat carbon black 34 34 48 34 34 34 34 34 34 34 34 sensitive dispersion butyral resin 27 19 22 27 27 27 66 27 mask layer binder polyamide containing a tertiary 39 47 30 66 composition amine group polyamide containing an ether 39 group polyamide containing a quaternary 39 ammonium salt group polyamide containing a sulfonic 39 group polyamide containing no polar 36 66 group polyvinyl alcohol 66 results of light blocking effect 2.4 2.3 2.8 2.4 2.4 2.1 1.7 2.3 1.6 1.5 2.3 evaluations durability ∘ ∘ Δ ∘ ∘ ∘ ∘ x ∘ ∘ x

As clearly indicated by Table 1, in Examples 1 to 6 in which butyral resin and polyamide containing polar group were used in combination as a dispersion binder for carbon black of a heat sensitive mask layer, superior properties were obtained in both of light blocking effect and durability, although a thin film having a film thickness of 1.5 μm was used. Here, in the case of Comparative Example 3 in which a butyral resin and polyamide containing no polar group were used in combination, the light blocking effect was extremely lowered, although the durability was superior. For this reason, in the case of a printing original plate using the heat sensitive mask layer of Comparative Example 3, the layer thickness of the heat sensitive mask layer has to be made thicker so as to allow the heat sensitive mask layer to have a light blocking effect in such a level as to serve as a negative film. As a result, it becomes necessary to provide a high abrasion energy, and when the photosensitive resin layer in the printing original plate is flexible, wrinkles tend to occur on the photosensitive resin layer. Moreover, in the case of Comparative Example 2 in which a butyral resin is used alone as the dispersion binder and in the case of Comparative Example 5 in which polyvinyl alcohol is used alone as the dispersion binder, the durability is lowered although the light blocking effect is superior. Moreover, in the case of Comparative Example 1 in which polyamide containing polar group is used alone as the dispersion binder and in the case of Comparative Example 4 in which polyamide containing no polar group is used alone as the dispersion binder, the light blocking effect is lowered although the durability is superior. Therefore, in the case of a printing original plate using each of the heat sensitive mask layers of Comparative Examples 1 and 4, the layer thickness of the heat sensitive mask layer needs to be made thicker in the same manner as in the printing original plate using the heat sensitive mask layer of Comparative Example 3, resulting in problems of high abrasion energy and wrinkles on the photosensitive layer.

As indicated by the above-mentioned results, it is considered that by using a butyral resin and polyamide containing polar group in combination as a dispersion binder for carbon black of a heat sensitive mask layer, it becomes possible to obtain a printing original plate having a heat sensitive mask layer having high light blocking effect and durability yet prepared as a thin film.

Industrial Applicability

Since the photosensitive letterpress printing original plate of the present invention has high light blocking effect and durability although it has a heat sensitive mask layer with a thin thickness, it is effectively useful, in particular, for a CTP plate that is subjected to abrasion by an infra-red laser. 

The invention claimed is:
 1. A photosensitive letterpress printing original plate including at least (A) a supporting member, (B) a photosensitive resin layer, and (C) a heat sensitive mask layer that are successively laminated, wherein the heat sensitive mask layer (C) contains carbon black and a dispersion binder, and the dispersion binder comprises a butyral resin and a polyamide containing a polar group selected from the group consisting of polyamide containing a tertiary amine group, a polyamide containing a quaternary ammonium salt group, polyamide containing an ether group and polyamide containing a sulfonic group.
 2. The photosensitive letterpress printing original plate according to claim 1, wherein the butyral resin and the polyamide containing polar group have a weight ratio in a range from 20 to 80:20 to
 80. 3. The photosensitive letterpress printing original plate according to claim 1 or 2, where the carbon black and dispersion binder have a weight ratio in a range from 25 to 50:20 to
 75. 4. The photosensitive letterpress printing original plate of claim 1 , wherein the heat sensitive mask layer has an optical density of at least
 2. 5. The photosensitive letterpress printing original plate of claim 1, wherein the heat sensitive mask layer has a thickness of 0.5 to 2.5 μm. 